The invention relates to an electric lamp provided with:
a concave reflector with a main shape which reflector has an axis of symmetry;
a lamp vessel which is closed in a gastight manner, in the reflector;
a light source having a first and a second light-generating portion in the lamp vessel, said first and second portion being arranged next to one another and on either side of the axis of the reflector, and being connected to current conductors which issue from the lamp vessel to the exterior at an end of said vessel.
Such an electric lamp is known from FR-1 096 668.
The known electric lamp is an incandescent lamp with two filament portions arranged in parallel, which lamp is placed in a separate spherical reflector. The lamp is particularly suitable for use as a motor vehicle headlamp, according to this patent document. The known lamp has the disadvantage that it supplies a light beam with two maxima in the luminous flux, as is shown in FIG. 4, and that it accordingly provides two spots of equal brightness lying next to one another at a distance from one another on a screen placed transversely to the beam. This is not acceptable for a lamp designed for providing accent lighting.
It is indeed possible to make a light beam with only one maximum, but in that case the light source must be linear, for example arranged on the axis of symmetry. This has the disadvantage that the lamp will have a comparatively large dimension then if suitable for operation on mains voltage, and that the reflector must also be bulky. The reflector must be deep in the case of an axial positioning of a linear light source so as to be able to surround the light source substantially completely in lateral directions. In addition, a current conductor must then be returned alongside the light source, which causes a shadow. In a transverse arrangement, the reflector has to be very wide, and the light beam will have an elongate, comparatively low maximum.
It is also possible to make a light beam with only one maximum by positioning the filament in a multiple-fold arrangement. This, however, implies a complicated construction of the lamp and a comparatively low luminous efficacy which is caused by the many comparatively cold spots in the filament arising from the supports necessary for keeping the filament in its folded state.
It is an object of the invention to provide an electric lamp of the kind described in the opening paragraph which is suitable f or creating accent lighting, which is of a simple construction allowing a compact shape, and which supplies a light beam with substantially only one maximum also when operated on mains voltage.
According to the invention, this object is achieved in that the reflector has a first and a second reflector portion,
the first and the second reflector portion each have an optical axis substantially parallel to the axis of symmetry and each surround their respective optical axis substantially, and
the first and the second light-generating portion coincide at least partly with said first and second optical axis, respectively.
In the lamp according to the invention, the first and the second light-generating portion of the light source each have their own reflector portion which substantially surrounds the light source portion and shapes the light from the respective portion into a beam. The beam portions coincide substantially and form one light spot with one maximum in the illuminance on a screen placed transversely to the beam. Light not radiated by a light source portion to its own reflector portion but hitting the other reflector portion will not be lost, but will be added to the beam. This light contributes to a gradual decrease in the luminous flux in directions enclosing an increasing angle with the axis of symmetry. Added to this is the light which hits the screen without previous reflection by being directly radiated in all directions away from the reflector by the light emission window of the reflector. The proportion of this non-reflected light depends on the value of the spatial angle with which the reflector surrounds the light source. The greater this spatial angle, or in other words: the deeper the reflector, the smaller the proportion of non-reflected light. It is favorable for achieving a comparatively high luminous flux in the beam if the reflector surrounds the light source at least substantially in lateral directions.
The lamp according to the invention will generally have a light beam with a substantially smooth contour from its maximum to its half-value directions, i.e. directions with respect to the axis of symmetry, in which the luminous flux has half the value as compared with the direction of the axis of symmetry, while local sub-maxima are avoided.
Since the reflector portions each substantially surround the associated light-generating portions of the light source, their optical axes have a mutual distance and half this distance to the axis of symmetry. In general, the mutual distance of the optical axes lies between a few and a plurality of mm, for example between 2 and 8 mm. This arises inter alia from the fact that the light-generating portions of the light source are comparatively close together so as to render it possible to introduce the light source into the lamp vessel during lamp manufacture and to obtain a comparatively high luminous efficacy in that heat losses are reduced by placing the portions close together.
The reflector portions may have various shapes. They may be, for example, paraboloidally curved. Alternatively, they may each have a body of revolution of a curve which is composed of several parabola portions. The reflector portions may be smooth, or segments, for example axially directed segments, may be superimposed thereon. The segments may be planar or convex transversely to the optical axis, depending on whether a comparatively narrow, a wider, or a comparatively wide light beam is desired. Planar segments are usually favorable, also for obtaining a tolerance as to the position of the light-generating portions of the light source. Alternatively, the reflector portions may be faceted.
The light source may be an incandescent body, preferably in a gas comprising halogen for obtaining a high luminous efficacy and lumen maintenance during lamp life in that a tungsten-halogen cycle takes place in the lamp. The lamp may in that case be made from quartz glass, hard glass, or a ceramic material, for example sintered aluminum oxide.
The light-generating portions of the light source may be arranged substantially parallel to one another, for example each coinciding substantially with the optical axis of a respective reflector portion. Alternatively, the portions may be in a V-shaped arrangement, for example with the bottom of the V being the part of the portions which is the closest to the light emission window. It is favorable in this arrangement that only one support is necessary for it. With only one support, which is T-shaped at its supporting end, the light source may alternatively be substantially U-shaped, for example with parallel or diverging legs.
The lamp vessel of the lamp according to the invention may have a shape and a reflectorizing layer, for example at an outer surface, such that the lamp itself supports the reflector. Alternatively, if the lamp is a halogen incandescent lamp, the lamp vessel may be accommodated in an outer bulb, for example blown, for example from glass, which bulb is shaped and reflectorized so as to constitute the reflector.
The reflectorization of the lamp vessel or the outer bulb, as applicable, may be a metal layer, for example made of aluminum or silver, or alternatively may be a stack of alternating layers of high and low refractive index, for example Ta2O5, Si3N4 and, for example, SiO2, respectively, which stack forms a dichroic mirror.
The reflector may be a separate body in which the lamp vessel is provided. It is favorable if the lamp vessel is indetachably connected to the reflector. The light source may in that case be fixed in the reflector during lamp manufacture in an aligned state, so that the positions of the light-generating portions of the light source with respect to the reflector are secured. The reflector may be a metal body. It is favorable for electrical safety, however, if the reflector is an electrical insulator. It is favorable for the reflector to be a glass body for obtaining a comparatively high thermal load resistance, for example obtained in that molten glass is pressed into the desired shape in a mold and is provided with a reflectorizing layer, such as a layer mentioned above, for example on an inner surface.
In a favorable modification, the reflector has a light emission window which is closed with a disc. This embodiment provides an improved safety, for example as regards the prevention of adjacent combustible objects catching fire, for example if the lamp vessel should break. The disc may have an optical effect, if so desired, for example by means of prismatic structures or a roughened surface so as to direct or scatter emerging light.
In a further favorable modification which may be combined with the preceding one, the reflector has a circular, projecting rim around the light emission window. This modification has the advantage that the lamp has a round shape in front elevation, and thus resembles a conventional lamp in front elevation, merging into a round opening in a holder or luminaire in which the lamp is placed.
A lamp cap, for example an Edison lamp cap or a bayonet lamp cap, for example a bayonet lamp cap with contact pins which have thickened portions at their free ends such as those which are usual in the case of starters for fluorescent lamps, may be present at the reflector for fastening and contacting the lamp in a lampholder.